What Size Alternator Fuse ?

[GordMay]

I certainly endorse prudence & “doing it right”, but am uncertain that this “extra” load end (battery) fuse is actually prudent.

It will only provide extra protection in the event that BOTH the alternator fails open, AND a ground fault occurs in the cable between the alternator fuse and extra battery fuse (simultaneously). Though possible, this is a highly unlikely occurrence.

Although fault currents > 5,000A are possible from a solidly grounded (“bolted”) battery, they are actually very rare. Most faults are high resistance.
Notwithstanding, it will cost considerably more than ± $30 to purchase a fuse & holder rated at 5,000AIC; and more yet to install them. I’d guestimate the installed cost at more like $200+.

The 6 “extra” termination points (2 crimp terms, 2 bolts, 2 fuse clips) will add unnecessary resistance (hence voltage drop), and potential failure points, to the circuit.

If Aaron’s wire (alt to batt) is #8AWG or larger, he could install a 35 - 40A fuse at the alternator.
If the wire is only #10AWG, then the 30A fuse is appropriate.

[CharlieJ]

Gord-It is pretty rare that you and I are opposite sides of a safety issue but I will continue to disagree with your single OCPD premise.

It will only provide extra protection in the event that BOTH the alternator fails open, AND a ground fault occurs in the cable between the alternator fuse and extra battery fuse (simultaneously). Though possible, this is a highly unlikely occurrence.

Re-read my post on the alternator casualty that I described...the alternator internally faulted to ground. One casualty that provided a bolted fault.

Cost of adding an MRBF fuse and fuse holder to provide the additional OCPD to the alternator conductor AWG 8 (68 A ampacity in the E/R):

Fuse holder: W/M 9368895 $20
MRBF 60A fuse: W/M 9368937 $15
Ampacity of MRBF fuse: 10,000A
Number of additional terminations: 0
Source: http://www.westmarine.com

So I was pretty close in my estimate; it will cost the boat owner $35 to properly correct this critically important conductor in accordance with the latest ABYC Standards.

[jdoe71]

I had a 150 amp breaker on the alternator, also have them on both positive lines at the battery banks.

I took the one off the alternator because I am reasoning that if this breaker opens there will be no place for the alternator output to go which will fry the diodes and put the alternator out of commission. I have seen breakers open on their own for no apparent reason, or someone accidentally hit the test switch that opens the breaker.

I felt that with breakers at the battery banks, if one opens accidentally the alternator still has someplace for output to go, but done the other way around it is risking frying an expensive alternator.

Faulty reasoning?

[cal40john]

Is your harness original wiring? The Yanmar manual I have shows the starter connected to the alternator with no fuse in that line. (In a minimum no frills install the battery is connected to the starter.) The 30 amp fuse is in a another wire that branches off from there that goes to the panel and supplies power via the instrument key switch to all engine electrical functions, lights, horns, relay to engage starter.

So the fuse does not carry the alternator output to the battery if it is an original wiring harness. The fuse is sized for starter solenoid operation, field coil of alternator and lights/horn.

As an aside, if you haven't done any extra wiring as my boat was wired, the alternator output goes through (under a foot) of about #10 gage to the starter, where the large gage wire to the battery takes over.

John

[btrayfors]

IMO, fuses are much to be preferred over breakers in alternator lines. As was noted above, CB's fail -- both closed and open -- and I've seen several that did. Fuses tend not to fail, but do their thing and do it fast.

A single fuse in the alternator-to-battery positive line will provide overcurrent protection, whether the fault be at the battery end or the alternator end. I'm with Gord on this; use a fuse sized for the maximum output of the alternator....for most installations.

The only "exposure" remaining is the cable itself. If somewhere between the battery and the fuse it were to become bare and shorted to ground, this could set up a rather horrible situation. If you think there's any chance of this happening, then put another fuse near the battery.

The 7" rule, again IMO, is there to protect against such eventualities as the wire itself being shorted, not necessarily the device at the other end. This belief is supported by the 40" and 72" exceptions #2 and #3 (if the wire is in a conduit or is similarly protected throughout its length), then the CPD may be located up to 40" from the power source or 72" from the battery.

Many boats have the alternator output connected to the starter solenoid. If the run is less than 40" overall and the wire is protected throughout its length, then no CPD is required (exception #4).

Hey, I'm as paranoid as the next guy when it comes to wiring on a boat, but IMHO two fuses in the same line with little or no chance of itself being shorted out is overkill. If a short develops at either end, the fuse will blow.

Bill

[CharlieJ]

$35...wow, what a lot of words over $35.

[GordMay]

Originally Posted by CharlieJ

... Re-read my post on the alternator casualty that I described...the alternator internally faulted to ground. One casualty that provided a bolted fault...

A bolted fault is basically a solidly connected zero impedance dead short, with no arcing.

This is as opposed to an arcing fault where the conductors are simply/barely touching.

Bolted fault conditions are not common when functioning systems are properly maintained or repaired.

I have no doubt that we share the same objectives - a safe & effective installation.

[CharlieJ]

A bolted fault is basically a solidly connected zero impedance dead short, with no arcing.

This is as opposed to an arcing fault where the conductors are simply/barely touching.

I understand the difference. What I described was a bolted fault.

Bolted fault conditions are not common when functioning systems are properly maintained or repaired.

Absolutely agree. What I described was a very uncommon condition. It did; however, occur.

I have no doubt that we share the same objectives - a safe & effective installation.

I know we do.

[CharlieJ]

Bill-
You were a bit selective in describing E-11.10.1.1.1 Exceptions:

Many boats have the alternator output connected to the starter solenoid. If the run is less than 40" overall and the wire is protected throughout its length, then no CPD is required (exception #4).

Exception #4 is only valid if-
1. The alternator is self limiting, and
2. has an integral regulator, and
3. the conductor is less than 40" long, and
4. is connected to a source of power other than the battery, and
5. the conductor is contained throughout its entire length in a sheath or enclosure.

[Maine Sail]

I'm confused.

I run my alternator directly to my house bank. This avoids fried diodes. The house bank is fused within about 7" or less. I simply tied my alternator cable into the load side of the fuse at the fuses post. Simple, no extra fuse as you already have one on the house bank. I still have a fuse at the alt end but just used the house bank fuse as my battery end fuse for the alt. If you do this though the wire should be able to handle the current so that it can blow the fuse before a fire ensues.. A 250 amp fuse on a 12 ga alt wire (seen it) is probably not such a good idea..

The only reason I would want a fuse at the batt end of the cable is for a dead short in the alt wire before it gets to the alt fuse. Having been on a boat during a race that suffered a dead short with no fuses I am a believer in over over current protection. I watched a mutistranded wire become a solid mass & a one condutor wire in mere seconds. The jacket was literally vaproized.. Scared the hell out of us..

If your alt cable is well secured, and the potential for chafe is near zero, and it's a relatively short run, you are probably safe but if you have a large enough alt wire why not just use the house bank fuse as the battery end of your alt wire?

[jdoe71]

Our alternator is wired directly to the house batteries, there are two banks of two 6V batteries in series in each bank. Each bank has a 150 amp breaker on it at the batteries. We have 90 amp alt with external reg. Separate starter battery is charged by echo charger. We don't use lot of power, probably 60 to 80 amps per day from 460 amp bank. Battery wiring is 1AWG.

I felt that it was unlikely that each breaker would open accidentally at the same time, therefore, there should always be a path for the alternator to the batteries. Hopefully would avoid fried diodes since we always leave the switch on All so both banks are seen by the alt all the time. Left the switch in since I wanted to be able to exclude a bank if I lose a cell in a battery.

If they both opened at once there's something wrong for sure and I suppose blowing the diodes isn't the end of the world since we have a spare alt but these things often happen when you least need it to.

[CharlieJ]

As we wander through the forest, let's not forget that the intent of over current protection devices (OCPDs), in the context of ABYC E-11 is to protect the conductor from current in excess of the conductor's capacity (ampacity) to safely conduct that current without exceeding the temperature rating of the insulation of the conductor.

If a properly sized OCPD (I prefer fuses, MRBFs to be exact, over circuit breakers in this application) opens, you have probably saved your boat. Fried diodes are the least of your worry and can be mitigated by putting a zap stop from B+ to Vessel Ground to shunt the high voltage spike that occurs when the alternator output opens.

[Maine Sail]

Originally Posted by CharlieJ

As we wander through the forest, let's not forget that the intent of over current protection devices (OCPDs), in the context of ABYC E-11 is to protect the conductor from current in excess of the conductor's capacity (ampacity) to safely conduct that current without exceeding the temperature rating of the insulation of the conductor.

If a properly sized OCPD (I prefer fuses, MRBFs to be exact, over circuit breakers in this application) opens, you have probably saved your boat. Fried diodes are the least of your worry and can be mitigated by putting a zap stop from B+ to Vessel Ground to shunt the high voltage spike that occurs when the alternator output opens.

If the one fuse you are using for both the load wire and the alt wire protects both of them then why not use the same fuse for both?

Personally, I prefer my alt running directly to the bank rather than using a Zap stop. By doing this on my boat I am not also going through the starter stud and the battery switch, 3 extra mechanical connections, while doing so.

[CharlieJ]

If the one fuse you are using for both the load wire and the alt wire protects both of them then why not use the same fuse for both?

1. Not in accordance with ABYC Standards.
2. Fuses (and circuit breakers) are not to be continuously subjected to >80% of their rated ampacity. With the example used of AWG 8 wire, a 60A fuse would be appropriate. You would only have 0.8 x 60A = 48A that could be run through that fuse. With the alternator at full capacity of 30A, you would only have 18A available for the rest of the vessel without over stressing the fuse.

Personally, I prefer my alt running directly to the bank rather than using a Zap stop.

Sorry, I do not understand. The Zapstop is typically made up of a diode and a fuse holder with AWG 10 wiring. It is placed between the output stud of the alternator and ground and will provide a path, through the magic of semiconductor behavior, to provide a path to ground of the high voltage spikes that occur when the output from the alternator is opened.

By doing this on my boat I am not also going through the starter stud and the battery switch, 3 extra mechanical connections, while doing so.

If you have terminations on each end of your alternator output conductor, and if you use the MRBF that I discussed in a previous post, then you are adding two mechanical connections; i.e., where the fuse holder bolts to the stud.

Each MRBF setup costs $35 (current price) at West Marine.

There has been a lot of discussion about spending $35 to properly and adequately protect a potentially high fault current circuit.

[GordMay]

Originally Posted by CharlieJ

... If you have terminations on each end of your alternator output conductor, and if you use the MRBF that I discussed in a previous post, then you are adding two mechanical connections; i.e., where the fuse holder bolts to the stud...

Not to nit pick, but the MRBF adds 3 connections:
1 at each end of fuse + 1 at fuseholder lug.